1. Field of the Invention
The invention herein relates to photoluminescent metal-ligand complexes. More particularly it relates to novel complexes having extended luminescent lifetime and that are useful as optical oxygen sensors.
2. Description of the Prior Art
Photoluminescent molecules that are based on transition metals have been studied extensively for a number of years. The chemistry and photophysics of these molecules have been thoroughly investigated because of the varying potential applications for the photoluminescent transition-metal complexes. The most widely studied photoluminescent complex has been the tris(2,2xe2x80x2-bipyridyl) complex of ruthenium(II), written as [Ru(bpy)3]2+. The primary properties of [Ru(bpy)3]2+that are important for most applications are a long excited-state lifetime, strong absorption in the visible region, and a high quantum yield of luminescence.
Although many applications of [Ru(bpy)3]2+ have been investigated, the high cost and low availability of ruthenium are a substantial impediment to large-scale applications of the molecule. Research to find cheaper alternatives to [Ru(bpy)3]2+, such as complexes based on copper, have been investigated, but none have been identified which have the long excited state lifetimes and high quantum yields of luminescence that are observed for the ruthenium-based complexes.
We have now invented a new class of metal-(bis)ligand complexes which have quantum yields of luminescence that are comparable to that of [Ru(bpy)3]2+. In the solid state, our complexes display excited-state lifetimes that are longer than that observed for [Ru(bpy)3]2+. Because of the cost differential between copper and ruthenium, our complexes cost substantially less than [Ru(bpy)3]2+. These new complexes are anticipated to generate significant large-scale applications which have heretofore not resulted from the [Ru(bpy)3]2+ complexes.
Specifically, the complexes of this invention are based on copper, silver, zinc and lithium, and those other metals, particularly transition metals, which may be found to exhibit complexing properties equivalent to the named metals. (We recognize that lithium as a Group 1 element is commonly classified as an xe2x80x9calkali metalxe2x80x9d rather than as a conventional metal of the Groups 3-12. In the present invention, however, lithium exhibits the same complexing properties as copper, zinc and silver, and will therefore be considered to be within the definition of xe2x80x9cmetalxe2x80x9d for the purposes of this invention.)
The complexes of the present invention are (bis)ligand, with the two ligands being different from each other. Preferably the ligands are each a substituted phenanthroline. A wide variety of substituents are possible, subject to the limitation that no significant adverse reactions competitive to the desired complexing reaction are promoted by a particular substituent. Substituents may be alkyl, aryl or alkaryl hydrocarbons, and the hydrocarbons may also be substituted with halogen, chalcogen, nitrogen or phosphorus moieties, or contain heteroatoms. The specific substituents which will work will depend on the steric properties of the pair of substituents being considered. We have found the necessary steric considerations are that one of the substituents must be of a size and shape that prevent a pair of that substituted ligand from complexing with the metal atom (M).
The method of formation of such complexes is also an aspect of the present invention.
Further description and representative examples will be presented below.